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Simple Semi-Auto

A place to ask general spud cannon related questions.
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Simple Semi-Auto

Unread postAuthor: TeyBobster » Wed May 26, 2010 11:21 pm

I like making things simple, which means more reliable.

I just got into pneumatic spud cannons and I think its awesome! So I was looking at a lot of semi-auto designs and it annoyed me how you would have a dry-fire 2 out of 3 shots. Sure these were fast, but I wanted reliable :o

This probably has so many issues in so many different areas, but I thought hey, you guys are professions, why not give it shot and see if you can help me out?

P.S: The piece of PVC that lets the ammo in is attached to the piston. This is not to scale all either.

P.P.S: There's a lot in the drawing that doesn't work out but bear with me; this is just a concept that I drew up really quick. The length of the stopper would have to be just right to let a ball out after all the pressure would be released. Also, the area behind the piston, would have to be the right length.
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Attachments
scan0004.jpg
The dots are the leaks. This design makes the top chamber redundant(useless) though.
scan0005.jpg
Version Two!
I would have to add some strong supports for this to work...
mountfix.jpg
Coaxial design is non é coretto! I just done like how a separate chamber looks. This would require some extra cutting.
mountfix.jpg (5.82 KiB) Viewed 1333 times
Last edited by TeyBobster on Thu May 27, 2010 12:26 am, edited 3 times in total.

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Unread postAuthor: jor2daje » Wed May 26, 2010 11:33 pm

If you have airflow in behind the piston your trying to pilot, you wont create anywhere near as much of a pressure differential on the piston so it will either actuate really slowly or wont actuate at all. The solution would be to either use another ball valve which wouldnt be real semi-auto, or use a dcv which adds complexity.

Also the chambers volume may be emptied, making the piston close before ammo has time to drop in.

And if its a coaxial your drawing it will be incredibly difficult to get the magazine to seal with the chamber and the barrel.

Having just built one, I can tell you using a dcv and pneumatic cylinder may seem more complex but in the long run it simplifies many more problems.

p.s. its p.p.s ;)
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Unread postAuthor: TeyBobster » Wed May 26, 2010 11:57 pm

Thank You! You're insight has been very helpful! I made a second version. I will try to refute your issues :P

  1. making the piston close before ammo has time to drop in: This would be operated at a low P.S.I, causing the piston seal slower, giving enough time for the ammo to drop in. If I did the math, I think I can come up with several solutions such as a bigger chamber behind the piston.
  2. it will be incredibly difficult to get the magazine to seal with the chamber and the barrel: Very true, and to make sure, the magazine is circular, which I would think would make it easier to seal.
  3. actuate really slowly or wont actuate at all: It will operate very slowly, simplicity and reliability is more important in this design than function.

Thank You again, for you insights.
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Unread postAuthor: jor2daje » Thu May 27, 2010 12:06 am

you still will need either a really small amount of flow from the compressor or whatever to make sure you pilot decreases pressure enough to actuate the piston. You could use a needle valve or something but then you chamber would take a long to to fill and you piston would take longer to seal.
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Unread postAuthor: deathbyDWV » Thu May 27, 2010 12:09 am

K

edit... Strange, my post dissapeared and all that was left was a "k"...:?
Here's what I was goin to say...

If you want really reliable and not that fast, have you considered a mag fed pump action of sorts?

And to fix the comp flow issue you could just have an extra big pilot...
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Unread postAuthor: TeyBobster » Thu May 27, 2010 12:14 am

jor2daje wrote:you still will need either a really small amount of flow from the compressor or whatever to make sure you pilot decreases pressure enough to actuate the piston. You could use a needle valve or something but then you chamber would take a long to to fill and you piston would take longer to seal.


Sorry, I didn't quite follow that. I'm not sure if this has anything to do with your question: there is a ball valve that releases the pressure from the pilot, I just twist it open when I want to fire it, and then twist it shut once I hear the ball drop into the barrel.
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Unread postAuthor: TeyBobster » Thu May 27, 2010 12:16 am

deathbyDWV wrote:K

edit... Strange, my post dissapeared and all that was left was a "k"...:?
Here's what I was goin to say...

If you want really reliable and not that fast, have you considered a mag fed pump action of sorts?

And to fix the comp flow issue you could just have an extra big pilot...


You are exactly right! That is part of my design, I forgot to mention it in the original post. Can you give me a link to a post of the "mag fed pump action"?
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Unread postAuthor: jor2daje » Thu May 27, 2010 12:21 am

Ok, a piston works by differential of pressures, there is a larger surface area on the pilot side so the same pressure creates a higher force, pressing the piston against the seat. When you open your pilot valve the pressure is vented from behind the piston making it so there is more force acting from the chamber, thus opening the piston allowing air out of the barrel. The greater the pressure differential created the faster the opening time. If you have air coming into the pilot at the same time as your try to release pressure from the pilot area the pressure will remain constant and air you add will simply escape through the pilot valve. This means less pressure differential and a slower opening valve.
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Unread postAuthor: TeyBobster » Thu May 27, 2010 12:38 am

Ah, I see and no size of ball valve would allow me to release air faster than it was pumped in?

Mathematically, this could work? Extended section is the same size as chamber.
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Unread postAuthor: jamie e » Thu May 27, 2010 1:48 am

I had a very similar design, the only differences are: mine is a 'T' valve and could go full auto if you swaped the pilot valve with a pop-off valve.
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Unread postAuthor: jor2daje » Thu May 27, 2010 10:51 pm

No a most ball valves would vent air faster than its coming in the point is there is more pressure behind the piston if more air keeps coming in as you vent it.
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